Not quite compiler independent, but we've never tried to be gcc independent

anyway.  This is at least less ugly than what was there before, and fixes
building all sources at once.

include/libbb.h

@@ -414,12 +414,6 @@ int is_in_ino_dev_hashtable(const struct stat *statbuf, char **name);
 void add_to_ino_dev_hashtable(const struct stat *statbuf, const char *name);
 void reset_ino_dev_hashtable(void);
 
-/* Stupid gcc always includes its own builtin strlen()... */
-extern size_t bb_strlen(const char *string);
-#ifndef BB_STRLEN_IMPLEMENTATION
-#define strlen(x)   bb_strlen(x)
-#endif
-
 char *bb_xasprintf(const char *format, ...) __attribute__ ((format (printf, 1, 2)));
 
 #define FAIL_DELAY    3